Flexographic printing is a method of direct printing similar to letterpress that uses resilient relief-image plates, sleeves or cylinders of rubber or photopolymer material.
Historically the plate preparation has progressed from hand carving to imagewise photopolymerization to laser engraving. Whichever method of printing member preparation is used the finished product suffers from reduced print quality due to highly localized pressure during the printing operation. U.S. Pat. No. 3,425,347 (Nard et al.) suggests that the effect is caused for instance by “inaccuracies inherent in a rubber plate”, or the center of a printing area being pulled down during formation (the defect being known then as “cupping”). As a remedy, the inventors suggest the use of a patterned cushion attached to the underside of the print surface.
U.S. Pat. No. 3,903,794 (Grupe et al.) describes a resilient foam which is cast onto a support, ground down to give uniform thickness cushion, and then attached to the printing member.
U.S. Pat. No. 4,574,697 (Feely) describes a foam mounted on a base film where the film and the foam are both coated with adhesive. The base film is bonded to the printing press cylinder and the foam is bonded to the back side of the imaged flexographic printing member. The invention is not so much concerned with the cushioning effect of the foam as with the retention of integrity of the foam/base on removal from the print cylinder after printing.
U.S. Pat. No. 5,325,776 (Rather Sr. et al.) claims improved cushioning relative to previous cushions that had suffered from lack of deformability or if they were readily deformable, from a lack of sufficient resiliency to rebound rapidly enough and repeatedly to the original dimensions. Also, most materials were not sufficiently accurate in caliper to give uniform print quality. The cushioning material disclosed contained closed cells and elastomeric particles dispersed in, for instance, a polyurethane rubber.
U.S. Pat. No. 5,894,799 (Bart et al.) claims that the closed cells of a cushion tend to break on successive use such that the cushioning material shows fatigue, loses compression and resilience qualities and thus the print quality deteriorates. Bart et al. describes an open cell structure for the cushion with a minimum total void of 40 percent. Bart et al. discloses the application of cushions to flexographic printing members imaged by laser engraving.
U.S. Pat. Nos. 6,247,403 and 6,666,138 (both to Randazzo) suggests a different type of cushion. Instead of the foam structure of the cushions described in earlier patents, Randazzo uses patterns of protrusions. U.S. Publication No. 2009/0211480 (Castillo et al.) describes cushions with low friction surfaces to improve printing.
Recent developments in plate imaging have resulted in significant improvements in print quality and higher expectation from customers who for instance wish to use flexographic printing for high quality packaging applications. As described in the above publications, it has now become the general practice to attach the imaged flexographic printing member to a cushion. Pressure on the surface image of the flexographic printing member during printing, instead of causing image distortion, is taken up by slight compression of the cushion. This cushion may be selected from a range of cushions designed to fit the requirements of the imaged flexographic plate.
Tapes are manually bonded to the back surface of the imaged flexographic plate. This procedure has to be done carefully to avoid air pockets. A variety of tapes are sold to suit different plates and different types of images. Where one plate contains different types of image, either the image may be split up into text and pictures and extra sets of plates made to accommodate the split up images or a combination of tapes may be used to optimize print quality. When a print customer wants solid ink and crisp lines, the printer needs firm, high-density tape. When a printer needs to consistently balance solid and dot reproduction on the same plate, a wide range of combination printing tapes can be considered.”
Thus, different parts of the same flexographic printing member may benefit from different cushions. This can be done by cutting and sticking different types of tape corresponding to different areas of the plate, requiring additional manual manipulations. This type of problem was addressed by in U.S. Pat. No. 7,785,431 (Kuczynski et al.) by the incorporation of monomers into the plate precursor that could be used to selectively harden the plate in zones after imaging.
Another problem is that cushion thickness tolerance is much wider than that of the flexographic plate itself and that this introduces further challenges of unevenness.